1. Field of the Invention
The present invention relates to an image sampling and color shift correction system for preventing a shift of the transfer position of each color which is overlappedly transferred to a paper in a multiple image forming apparatus for forming a multiple image by providing a plurality of image output means along a transfer belt.
2. Description of the Related Art
Recently, the technology for coloring documents in a copy machine, a printer, a facsimile or the like has been rapidly advanced. These machines which deal with colored documents tend to have enhanced processing. An example of a machine with color processing of documents is a printer with four ROS (Raster Output Scanner), each with the colors black (K), yellow (Y), magenta (M), and cyan (C), i.e., a tandem color printer. However, since this tandem color printer uses a plurality of individual ROSes to form one image, when a user or a service man temporarily shifts a part of the image forming apparatus from the original image forming position to another position and then puts the parts back to the original position after fixing, for example, a paper jam or the like, and further, when the temperature has changed aged deterioration occurs, or something impacts on the apparatus, a subtle error occurs in the position relationships between the ROSes, photoreceptor drum and the transfer belt thereby resulting in the problem of the color shifting after the transfer of each color, i.e., register-shift occurs.
There are a shift component in the main scanning direction that the ROS scans, a shift component in the transfer belt carrying direction, i.e., in the sub scanning direction, and an angle shift component in the scanning direction of the ROS, i.e., a skew shift component or the like. Therefore, an image position recognition pattern predetermined by the respective ROSes is output from a pattern generator in accordance with a constant rule, the image position recognition pattern is transferred to the transfer belt and the pattern is sampled with a predetermined timing by change coupled devices (CCDs) disposed downstream of each image forming apparatus.
The CCDs sample the position of an image for respective colors. The difference between the sampled positions and the expected positions for a system with no color shifts of the image position recognition patterns for predetermined respective colors is detected, and the amount of the register shift of the respective colors is calculated from the detected data. A method for providing a little register shift and a high quality picture by correcting the ROS's write timing or the like in accordance with the calculated results is well known (for example, Japanese Patent Unexamined Publication (Kokai) No. 63-279272). However, in the above-mentioned conventional method, the register fitting cycle is differentiated from a usual job cycle and a register shift measurement pattern 101 is written in an image area 103 of a transfer belt 102, as shown in FIG. 9. Accordingly, there arise the following problems.
1) Since a special cycle is provided to correct the color shift, a time loss occurs.
2) Although a best color register state is obtained directly after the correction of the color shift, the color register state is gradually deteriorated for reasons of temperature change or the like after the correction.
3) A register shift due to an external force (for example, an impact or leaning) which cannot be detected by the machine cannot be timely found.
4) A check cycle in which the amount of the color shift is detected has a poor accuracy because of the low level of sensing. Therefore, if the picture quality and the color register have significantly deteriorated it is judged that the color shift is present, whereby a correction cycle is not executed. Consequently, the print is output as it is without the correction of the color shift, until a state occurs in which a color register shift is extensive.